Well control system



' Aug. 5, 1947. P. E. LEHR 2,425,193

WELL CONTROL SYSTEM Filed Dec. 26, 1944 2 Sheets Shet 1 Swim 1 0mm; Table Aug. 5, 1947. P. E. LEHR 5, 3

WELL CONTROL SYSTEM v Filed Dec. 26, 1944 2 Sheets-Sheet 2 Swivd IOO / Rump I90 lnverfior: Paql E. Lzhr I Patented Aug. 5, 1947 WELL CONTROL SYSTEM Paul E. Lehr, Bakersfield, Calif., assignor to Shell Development Company, San Francisco, Calif., a corporation of Delaware Application December 26, 1944, Serial No. 569,864

6 Claims.

This invention pertains to the drilling of deep wells, and relates more particularly to a method and apparatus for use in controlling high pressure wells during drilling and completing operations.

It is well known that, in general, the zone or rock pressure encountered at any particular depth in a well is approximately equal to the hydrostatic head of a column of water having the same height, so that it can be roughly calculated from the formula wherein N is the depth in feet, and P is the formation pressure in pounds per square inch at said depth.

It is, however, also well known that in the case of so called high pressure wells, the zone or rock pressure has a value considerably higher than the one obtained from the above formula. In order to balance said high pressures and to prevent blow-outs, it is customary in such cases to use drilling fluids having specific gravities considerably in excess of that of water. Thus, drilling fluids normally comprising, for example, a suspension of clay in water, may be loaded with weighting materials such as calcium carbonate, barytes, iron oxide, etc, to give them any desired high specific gravity up to or exceeding 2. The control of high-pressure wells while drilling with such Weighted fluids does not in itself present any appreciable difficulties.

It is however also generally true that the ideal drilling fluid for penetrating the producing zone proper is oil, such for example as a heavy crude oil, or an oil base drilling fluid, for example, a stabilized suspension of calcium carbonate in a crude or Diesel oil, such as described in U. S. Letters Patent 2,223,027, 2,222,949, 2,297,660 and 2,350,154. Oil and oil-base drilling fluids are free from the main drawback inherent to aqueous drilling fluids, which is that the water from the drilling fluid penetrates and plugs the producing zone, whereby the ultimate productivity of the well is unfavorabl affected.

'It has therefore lately been a common practice, upon reaching the top of the producing formation, to replace the aqueous drilling fluid with oil or an oil base drilling fluid, and to drill into the producing formation with said second. drilling fluid. In the case of ,high pressure wells, however, such procedure gives rise to considerable difliculties with regard to pressure control. When, for example, a relatively heavy column of aqueous drilling fluid circulating in the Well is replaced with a much lighter column of crude oil or of an oil base fluid, which oil :base fluid sometimes cannot be weighted to the required high specific gravity without excessive settling, there arises a considerable danger of blow-out during the well-completion period. Practices such as pressure completion, which consists in closing the Well at the top, and holding the formation pressures down by purely mechanical means, have therefore to be resorted to in such cases. Pressure completion, involving the use of special pack-off equipment, blow-out preventers, lubricators, etc., forms, however, a relatively difficult and complicated procedure, resulting in loss of time and increased costs.

It is therefore an object of this invention to provide a method for completing wells, and especially high pressure wells, without having recourse to conventional pressur completion procedure and equipment.

It is also an object of this invention to provide a method for completing wells wherein two drilling fluids having difierent specific gravities and preferably immiscible with each other are simultaneously maintained within the well during the completion period. It is also an object of this invention to provide a method for completing wells while regulating the back pressure on the well and balancing the well pressures by means of a liquid packer, said packer consisting of floating column of relatively high specific gravity drilling fluid.

It is also an object of this invention to provide the apparatus necessary for completing Wells by the present method.

These and other objects of the invention will be understood from the following description taken with reference to the attached drawings, wherein:

Fig. l is a diagrammatic view, partly in crosssection, showing one typ of apparatus suitable for use in racticing the present invention,

Fig. 2 is a similar view of another type of apparatus suitable for this purpose.

Referring to Fig. 1, a conventional swivel joint a drill string I! having a non-return valve 3| near its lower end, a drill collar 32 and a drill bit 33 provided with the customary orifices for the circulation of the drilling fluid. The swivel I is connected, through a conduit I and a pump 9, to a ditch or reservoir 8, filled with any desired drilling fluid, for example, a mineral oil, such as a crude oil, or an oil base drilling fluid The method of the present invention may now be described as follows:

The well is drilled in a conventional manner, and with circulation of a heavy drilling'fluid to the top of the producing zone 40. A string of casing 21 is then lowered into the well and the casing shoe 29 is cemented on top of said zone. Before drilling out the casing shoe the drill string I1 is temporarily withdrawn and a second or inner casing I5 is then lowered to any desired level, preferably within the upper portion of the well, within the first or outer casing 21, and is landed in a conventional manner at the surface. The length of the casing I5 is selected in accordance with many factors involved, such as the total depth of the well, bottom pressures to be controlled, density of the muds used, etc. In general, this length may be estimated as about one third of the depth of the well. The annular space or zone between the two casings is closed at the top as shown at IS. The upper end of the inner casing I5 may be left open as shown at I4, an overflow pipe 35 provided with a valve 36 leading to the heavy fluid reservoir. The annular space 23 between the two casings is in communication with bottom of the well must be overbalanced by the aback pressure valve I9.

The drill string is then lowered again into the borehole through the open end I4 of easing I5, no packers or pressure control equipment being necessary since the well pressure is kept down by the heavy fluid in the well. The pipe I is connected as shown in the drawing, the cementing shoe is drilled out, and drilling is carried on through the producing zone 40 while circulating oil or a light oil base drilling fluid through the well. It will be understood that at the beginning of this phase of the drilling operations, the well is still filled with the heavy drilling fluid with which the well had been drilled down to the top of the producing zone. When circulation, as described above, is switched to a light drilling fluid or oil, said oil is picked up by the pump 9 from reservoir 8 and is circulated upwards through pipes I I and I, downwards through swivel I, kelly 3, drill string I'I, bit 33, and upwards again through the annular space 23 which serves as a fluid passage, being finally returned, together with the cuttings it carries, to the reservoir 9, while a desired back pressure is maintained on the well or circulating fluid by means of valve I9. The oil or light drilling fluid, issuing into the well through the bit 33, and following lines of flow indicated by the arrows M, gradually forces out or carries out with it the heavy drilling fluid previously filling the well, until the whole zone or portion of the well below the inner casing I5, and the annular space or zone 23 are filled with said oil. The space or zone 42 within the inner casing I5, not being in the line of flow of the oil, remains filled with a quiescent column of the heavy drilling fluid, as indicated at 42, the tendency of said column to settle down by gravity being counteracted by the upwardly acting velocity 4 head of the circulating oil or light drilling fluid.

In order to provide for safe operating conditions, the formation pressure prevailing at the bottom of the well must be overbalanced by the hydraulic pressure of the liquid in the well which may be expressed as follows P hW+HL P (h1+H)L+B wherein:

P is the bottom hole formation pressure in lbs./

sq. in.

h is the height of heavy fluid column 42 within casing 23 in feet.

hl is the height of the light fluid column in the annular space between casings I5 and 2'! in feet.

H is the distance from the bottom of the well to the interface between the light and the heavy fluids (or substantially to the lower end of casing I5) in feet.

W is the weight of a cubic foot of the heavy fluid divided by 144.

L is the weight of a cubic foot of the light fluid divided by 144,

B is the back pressure in lbs/sq. in. applied by means of valve I9.

Since the two concentric liquid columns standing in the well must be in hydraulic equilibrium with each other, the above expressions may be combined as follows from which it will be obvious that the present method permits control of the height of the heavy fluid column by controlling the amount of back pressure applied by means of valve I9. In other words, the level of the heavy fluid within casing I5 will rise or fall as the back pressure is increased or decreased in accordance with well conditions. Meanwhile, the drilling through the producing formation may proceed in the regular manner, and the drilling string I! may be raised and lowered any desired number of times, for examplev to change drill bits or to perform other desired operations, without the complicated step of passing said drill string every time through the mechanical pack-off equipment required in conventional pressure completion, since said drill string is in the present case. freely raised or lowered through the heavy drilling fluid standing within casing I5 and serving as a variable level fluid packer to over-balance bottom hole pressures.

Current methods of mud control permit the use of high gel values in the heavy drilling fluid capable of preventing or minimizing its mixing with the light fluid or oil. Any of the heavy fluid carried out to the surface can be readily separated from the oils and if necessary, further amounts of heavy fluid may be added to the column 42 from the surface as required.

If it is found that the cuttings carried out with the light fluid or oil have a badly abrasive effect on the back pressure valve I9, this can be overcome by installing a trap 29 ahead of the valve I9.

Upon completion of the well, the inner casing I5 is removed, a liner is run in the regular manner into the outer casing 21, and the well is thereupon brought in.

Fig. 2 shows another installation according to the present invention, which may be preferable when it is desired to use larger bit sizes. In Fig. 2, elements identical with those of Fig. 1 are denoted by the same numerals.

It will be seen that this installation is quite similar to that of Fig. 1, except that the inner casing I5 is replaced by a tubular member I50 of a somewhat smaller diameter and extending through the rotary table 5 as a kelly, to the swivel l 80, from which it is supported.

The swivel I0!) is constructed to provide within it independent fluid communication from pump 9 and conduit 70 to the drill string I10, and, on the return branch of the flow circuit, from the tubular string I58 to conduit 210, provided with a back pressure valve I90, and to reservoir 8. The operation of this system is in all points similar to that of Fig.1.

I claim as my invention:

1. In a process for deepening a well by means of a rotary drill string, said well being filled with a first drilling fluid, the steps of establishing within the upper portion of the well two concentric zones surrounding the drill string, circulating through the drill string and one of said zones a second drilling fluid having a specific gravity less than said first drilling fluid, thereby displacing said relatively heavier fluid by said relatively lighter fluid from said zone and the space within the well below said two zones, retaining said relatively heavier fluid quiescent in the second zone as a variable-level fluid packer, and applying to the circulating system a back pressure sufficient to equalize the well bottom hole pressure with the pressures of each of the two fluid columns within the well, said first column comprising the relatively lighter fluid in the first zone and in the space within the well below the two zones, and said second column comprising the relatively heavier fluid in the second zone and the relatively lighter fluid in the space within the well below the two zones.

2. The process of claim 1, wherein the first drilling fluid has a specific gravity greater than one, and the second drilling fluid has a gravity less than one.

3. The process of claim 1, wherein the first drilling fluid is immiscible with the second drilling fluid.

4. The process of claim 1, wherein the first drilling fluid is a water base drilling fluid, and the second drilling fluid is an oil base drilling fluid.

5. The process of claim 1, wherein the first drilling fluid is a water base drilling fluid, and the second drilling fluid is a mineral oil.

6. In a process for deepening a well by means of a rotary drill string with the employment of a drilling fluid filling said Well, the steps of establishing within the upper portion of the well two concentric zones surrounding the drill string, the inner concentric zone being open at the top, lowering through said inner zone and the first fluid standing therein a drill string extending to the bottom of th well, circulating downwardly through the drill string and upwardly through said outer zone a second drilling fluid lighter than said first fluid, thereby displacing said heavier fluid by said lighter fluid from the space Within the Well below said two zones and from the outer zone, retaining the heavier fluid quiescent in the inner zone as a variable-level fluid packer, the downwardly acting gravity head of said quiescent heavier fluid being balanced by the upwardly acting velocity head of the circulating lighter fluid, and adjusting the level of the heavier fluid in the inner zone to balance the bottom hole Well pressure by regulating the back pressure on the circulating lighter fluid.

' PAUL E. LEI-IR.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,585,969 Ferguson May 25, 1926 2,043,504 Blow June 9, 1936 2,234,454 Richter Mar. 11, 1941 

